1. Field of the Invention
The present invention relates to a carrier for use in a two component developer developing an electrostatic image, and to a two component developer using the carrier and a toner. In addition, the present invention also relates to a developing device, an image forming method, an image forming apparatus and a process cartridge using the two component developer.
2. Description of the Related Art
Electrophotographic image forming methods typically include the following processes:
(1) Forming an electrostatic latent image on an image bearing member such as a photoreceptor (electrostatic latent image forming process);
(2) Developing the electrostatic latent image with a developer including a toner to form a toner image on the image bearing member (developing process);
(3) Transferring the toner image onto a recording material (transferring process); and
(4) Fixing the toner image on the recording material, resulting in formation of an output image (fixing process).
Recently, in the field of electrophotography, transition is rapidly being made from monochrome imaging to full color imaging, and the market for full color imaging is rapidly expanding.
In full color imaging, all the color images are formed by forming primary color toner images using four primary color toners, i.e., yellow, magenta, cyan and black toners, while forming secondary color toner images by overlaying two or more of the primary color toner images. Therefore, in order to prepare a full color image having a good combination of color reproducibility and clearness, the fixed color toner image preferably has a smooth surface to reduce light scattering at the surface. For this reason, color images produced by conventional full color image forming apparatus typically have a relatively high glossiness of from 10% to 50%.
With respect to the image fixing method, contact heat fixing methods in which a heated fixing member such as a heat roller or a belt is contacted with a toner image upon application of pressure thereto are widely used. Such contact heat fixing methods have advantages of fixing a toner image at a high speed and a high heat efficiency while imparting a good combination of glossiness and transparency to the fixed toner image. However, the contact heat fixing methods have a drawback in that they often cause an offset problem, in which a part of a toner image is adhered to a fixing member, and the adhered toner is transferred again to the image or another image, resulting in formation of an abnormal image, because the melted toner image is contacted with the fixing member upon application of heat and pressure thereto.
In attempting to prevent occurrence of the offset problem, typically fixing methods are used in which a fixing roller having a surface made of a material having good releasability such as silicone rubbers and fluorine-containing resins is used while applying a toner adhesion preventing agent such as silicone oils to the surface of the fixing roller. Although the fixing methods are effective in preventing occurrence of the offset problem, the methods have a drawback in that, since an oil applicator has to be provided, the fixing device becomes unacceptably large. Therefore, recent monochrome image forming apparatuses tend to use toner having a relatively high melt viscoelasticity and including a release agent in combination with an oil-less fixing device or an oil micro-coating fixing device, in which a small amount of oil is applied to a fixing member.
Similarly, oil-less fixing methods are often used for full color image forming apparatuses to miniaturize the fixing devices thereof and simplify the configuration. However, since full color image forming apparatuses preferably produce glossy images as mentioned above, color toners used therefor preferably have a lower viscoelasticity than toners used for monochrome image forming apparatuses, thereby increasing the chance of occurrence of the offset problem. Therefore, it is difficult for full color image forming apparatuses to use an oil-less fixing device.
In addition, toner including a release agent has drawbacks in that transferability of the toner to a recording material deteriorates because of having high adhesiveness to the surface of carrier, and a toner filming problem in that a film of toner is formed on the surface of the carrier used in combination of the toner, resulting in deterioration of the charging ability and durability (life) of the carrier
On the other hand, coated carriers in which a resin including a carbon black is uniformly applied on a core material thereof are provided in order to prolong the life thereof, i.e., to prevent occurrence of the toner filming problem and other problems such that the surface of the carriers is oxidized, the moisture resistance of the carriers deteriorates, the durability of the carriers deteriorate, the carriers are adhered to image bearing members, and the carriers damage and abrade the surface of image bearing members, and to control the polarity and quantity of charge of the carriers.
Although developers using such coated carriers initially produce images with good image quality, the image quality tends to deteriorate as the number of copies produced by the developers increases because the coated layer of the carriers is abraded. In addition, when the coated layer is abraded or the carbon black in the coated layer is released therefrom, the color tone of color images tends to change because the color toner used for forming color images is mixed with the abraded layer or the released carbon black. In attempting to prevent occurrence of such problems, titanium oxide, zinc oxide or the like is used as a substitute for carbon black. However, the resistivity decreasing effect of such materials is worse than carbon black.
There is a proposal for a coated carrier in which a cover layer including a needle-shaped particulate conductive material, an antimony-doped tin oxide (ATO), is formed on a core material. However, since such an antimony-doped tin oxide has a blue color, developers using such a coated carrier tend to cause the above-mentioned color tone changing problem.
There is another proposal for a coated carrier in which a resin layer including a particulate conductive material, in which a tin dioxide layer and an indium oxide layer including tin dioxide are overlaid on a base material, is formed on a core material. Since such a conductive material includes a rare metal, the coated carrier has a relatively high cost. In addition, rare metals have poor availability.
In addition, there is another proposal for a coated carrier in which a cover layer including a resin, a first particulate conductive material which is a particulate tin oxide having carbon thereon, and a second particulate conductive material which includes a particulate metal oxide and/or a particulate metal salt subjected to an electroconductive treatment.
Electrophotographic full color image forming apparatuses often produce pictorial images, which typically have a high image area ratio. When developers including a toner including an external additive in a large amount and a coated carrier are used for electrophotographic full color image forming apparatuses and images having a high image area ratio are continuously produced, there is a case in which the charging ability of the carrier deteriorates.
For these reasons, the inventors recognized that there is a need for a carrier (or developer) which can produce high quality images for a long period of time regardless of the image area ratio of the images and which hardly causes the above-mentioned problems such as the toner filming problem and the color tone changing problem.